1. Field of the Invention
The present invention relates to a process for producing 3-methyl-1-phenylphospholene oxide.
2. Prior Art
Phospholene oxides having a basic structure represented by the following formula ##STR1## are useful compounds which are used as a catalyst in various reactions. Use of 1-phenylphospholene oxide or 3-methyl-1-phenylphospholene oxide as a catalyst effective for conversion of an organic diisocyanate into a polycarbodiimide is disclosed in Japanese Patent Application Kokai (Laid-Open) No. 348934/1992, and use of 3-methyl-1-phenylphospholene oxide as a catalyst effective for conversion of an organic diisocyanate into a polycarbodiimide is disclosed in Japanese Patent Application Kokai (Laid-Open) No. 9252/1993.
Of the phospholene oxides having the basic structure represented by the above formula, 3-methyl-1-phenylphospholene oxide ##STR2## is thought to be advantageous in reactivity. The 3-methyl-1-phenylphospholene oxide having a double bond at the 3-position has been produced by a process represented by the following reaction formula, shown in Organic Synthesis. ##STR3##
That is, said 3-methyl-1-phenylphospholene oxide has been produced by reacting dichlorophenylphosphine (1) and isoprene (2) in the presence of an antioxidant to form a solid adduct (3), hydrolyzing the adduct, neutralizing the reaction mixture, extracting the neutralized mixture with chloroform, and drying the extract and subjecting it to distillation [Organic Synthesis, 43, 73 (1963)].
The above conventional process, however, has various problems. Since isoprene (a starting material) is incorporated inside the solid adduct, the solid adduct must be ground and the ground adduct must be washed with, for example, petroleum ether for isoprene removal before the hydrolysis step; this makes the process complicated and moreover there arises the leakage of isoprene vapor during grinding and washing, deteriorating the working environment.
With respect to the above conventional process, a problem of yield is also pointed out. That is, the yield remains as low as about 57-63% even when there are used such controlled reaction conditions as (a) isoprene is used in an amount of about 3 moles per mole of dichlorophenylphosphine and (b) the reaction is conducted at room temperature for 5-7 days or even longer. The yield becomes even lower when the reaction time is shortened or the amount of isoprene is decreased.
Thus, in order to obtain 3-methyl-1-phenylphospholene oxide in an economical yield by the above-mentioned process, isoprene must be used in excess. Consequently, isoprene is incorporated inside the solid adduct. This necessitates the grinding and washing steps for the solid adduct in the actual process for production of 3-methyl-1-phenylphospholene oxide.